Cisco 3750G switch, fans not working

b...@bfisk.demon.co.uk

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Oct 1, 2010, 3:50:17 AM10/1/10

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Hi,
I have one of these on my workbench and both the fans aren't working,
the switch itself is fine. On inspection it appears that an SMD
component has come off the motherboard, Z1. Whether this was due to
bad soldering or if it's been knocked I can't tell. There doesn't
appear to be any signs of physical damage or heat.
Tracing the circuit shows that the power to the fans was delivered by
this component and I would suspect it was a zener. Without the
original I wouldn't have a clue what the value was and these days it's
highly unlikely to be marked anyway!
I'm unable to find any servicing info on these boxes so in desperation
I'm hoping somone might have one they could look inside. The only
other 3750's we have are single fan units and the power arrangement is
different.
I've also left a note on the Cisco site but so far not received any
replies.

Thanks
Bob

Meat Plow

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Oct 1, 2010, 8:14:04 AM10/1/10

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If all else fails look for another power source. The fans should be
marked per what they require. Probably 12 volts # .3 amps? Even an
external wall supply wired into the switch to provide fan voltage isn't
too bad an idea.

--
Live Fast, Die Young and Leave a Pretty Corpse

PeterD

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Oct 1, 2010, 8:25:52 AM10/1/10

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How many volts are the fans? How many volts does the power supply
deliver? Same? Connect fans directly to powersupply... Different:
adaptive circuit to change voltage (regulator for example).

My bet is that they are the same...

nesesu

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Oct 1, 2010, 2:35:49 PM10/1/10

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On Oct 1, 12:50 am, "b...@bfisk.demon.co.uk" <b...@bfisk.demon.co.uk>
wrote:

Consider the possibility that the missing components was a chip fuse
or a fuse resistor. If one fan faulted, a fuse resistor could heat
enough to melt the solder, but it is unlikely it would drop off the
board until the solder lost all it's surface tension properties.
Likewise with a Zener. Has someone been in there before you? If not
the part may still ba floating around or stuck onto the board nearby.
I agree with others that the supply voltage and fan voltages are
likely the same.

Neil S.

whit3rd

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Oct 1, 2010, 6:16:22 PM10/1/10

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On Oct 1, 12:50 am, "b...@bfisk.demon.co.uk" <b...@bfisk.demon.co.uk>
wrote:

> I have one of these on my workbench and both the fans aren't working,

> the switch itself is fine. On inspection it appears that an SMD
> component has come off the motherboard, Z1. Whether this was due to
> bad soldering or if it's been knocked I can't tell. There doesn't
> appear to be any signs of physical damage or heat.
> Tracing the circuit shows that the power to the fans was delivered by
> this component and I would suspect it was a zener.

If it's in series with the fans, and they're 12V fans, you can reverse-
engineer
the component value. Usually, there WOULD NOT be a Zener in series,
because Zeners fail (typically) in short circuit; it'd more likely
be a limit resistor, or maybe even a thermal-sensing resistor with a
negative temperature coefficient.

I'd expect "ZD" or "CR" markings for a Zener diode.

I'd also expect the component to be rattling around inside the case
somewhere (get a hand lens and do the Sherlock thing...).

midibob

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Oct 7, 2010, 4:02:24 PM10/7/10

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Thanks for the replies guys, some good info there.
I didn't consider it might just be a fuse, but the low value sensing
resistor makes a lot of sense as that would enable the management
software to do its thing too.
The original part is nowhere to be seen so I might wait just a little
longer in case anyone comes up with the exact part otherwise its poke
and hope with some low value resistors.

Thanks
Bob